Conventionally, system-linked inverter devices have been developed with which power was supplied by linking DC power generated by a fuel cell, solar cell, or the like to a commercial power system. With this system-linked inverter device, a technique is proposed for reducing switching loss by reducing the number of times switching elements have to switch.
For example, Japanese Patent Application Laid-open No. H11-53042 discloses a technique for reducing switching loss by reducing the number of times switching elements have to switch in the region of practical use. Such reduction is achieved by setting the frequency of a triangular wave for generating a PWM (pulse width modulation) signal that controls the on/off operation of the switching elements to 20 kHz when the inverter output power Pout is not within a range of 30 to 80% of a rated value Pr, and changing the frequency of the triangular wave to a lower frequency (such as 15 kHz) when the inverter output power Pout is within a range of 30 to 80% of the rated value Pr.
Also, a method called current control by hysteresis is known as a method with which the number of times the switching elements have to be switched can be reduced more than with the method disclosed in Japanese Patent Application Laid-Open No. H11-53042.
This current control by hysteresis method involves generating a PWM signal by the method shown in FIG. 15, and controlling the on/off switching of the switching elements with this PWM signal.
In FIG. 15, the solid line curve A indicates the waveform of a control target value of a fundamental wave component of output current, while the dotted line curves AU and AD indicate the upper and lower limit waveforms within the allowable range when the fundamental wave component of the output current fluctuates. Also, the zigzag line B indicated by the one-dot chain line is the waveform of the current value outputted from an inverter device.
With current control by hysteresis, when the current value outputted from the inverter device rises to an upper limit value Iup of the allowable range ΔI, the level of the PWM signal is switched to the level at which the switching elements are controlled so that the supply of current power to the inverter is stopped (labeled “low level” in FIG. 15). On the contrary, when the current value outputted from the inverter device drops to a lower limit value Idown of the allowable range ΔI, the level of the PWM signal is switched to the level at which the switching elements are controlled so that current power is supplied to the inverter is stopped (labeled “high level” in FIG. 15).
Patent Document 1: Japanese Patent Application Laid-open No. H11-53042
Guidelines are proposed for linking a system-linked inverter device to a system. For instance, it is required to hold the effective value of the fundamental wave component (60 Hz in the Kansai area, and 50 Hz in the Kanto area) within a specific allowable range with respect to the output current, or to keep each of the fifth, seventh, and thirteenth order high-frequency components within 10 or less, and to keep them totally within 3% or less.
Examples of performance aspects generally required for an inverter device include higher output precision, faster response, and higher efficiency. Since the main object of a system-linked inverter device is to supply power to a system, unlike with an inverter device that is used to control a motor, the need for higher efficiency is given priority over higher output precision or faster response. Therefore, with a system-linked inverter device, on the condition that the above-mentioned guidelines be satisfied, it is preferable to increase efficiency by reducing as much as possible the number of times switching.
Current control by hysteresis is a method in which the current value outputted from a system-linked inverter device is kept as much as possible within an allowable range ΔI of the control target value (such as a control target value of ±3%) to lower the switching frequency of the switching elements and reduce switching loss. Therefore, from the standpoint of raising efficiency on the condition that guidelines be met, it is considered that current control by hysteresis is a PWM signal generation method that is more suited to a system-linked inverter device than the method described in Japanese Patent Application Laid-open No. H11-53042.
However, current control by hysteresis involves the following problems.
(1) A circuit is necessary to constantly monitor whether the AC current actually outputted from the system-linked inverter device departs from the allowable range or not.
(2) It is difficult to construct a unit in which a pattern of PWM signals is generated according to whether the AC current actually outputted from the system-linked inverter device departs from the allowable range, using a digital control system. Accordingly, good universality and flexibility cannot be taken advantage of in the design of the digital control system.